The present invention relates to a hot melt adhesive capable of forming a peelable adhesive bond and which is substantially composed of modified butene-1 homopolymer or copolymer or a modified ethylene homopolymer or copolymer. It is anticipated that this hot melt adhesive will be capable of exhibiting good polymer to polymer bonding. The present invention also relates to a laminar structure formed with the novel hot melt adhesive capable of forming peelable seals.
Hot melt adhesives can be formulated to be pressure sensitive but these adhesives are usually soft, tacky and have limited strength and adhesion. Conventional hot melts such as formulations of ethylenevinyl acetate copolymer (EVA), polyethylenes (PE), polyamides, or polyesters are rigid, and form good strong bonds to certain substrates and are generally incapable of being peelable.
Solvent applied contact adhesives provide good bond strengths but they require the use of solvents which can be either toxic, or pollutants or fire hazards. Further, few of these adhesives are capable of "peelability" as the term is later defined.
The novel polybutylene hot melt adhesive of the present invention is unique in that it requires no solvents, and has peelability, and is capable of bonding to a variety substrates.
A "peelable seal or bond" or a seal or bond having "peelability" is defined to be the seal or joint between two films or other types of substrates produced by either, heat sealing, impulse sealing, or application as a hot melt between two surfaces with the joint thus formed being able to be opened in the original plane joining the two films, by the action of a pulling force, without the opening of the joint causing wrenching off or tearing in the substrate(s) and the material from which the two films of the joint are made. For the purposes of the present invention, the peelable seal or bond must possess a mechanical resistance to opening or "peeling" which is sufficient to maintain a tight seal around an item or quantity of liquid or solid during storage and transport until a user of the packaged article wants access to the contents of the sealed packaging. The mechanical resistance of the peelable seal must be low enough to permit ready manual opening of the joint, i.e., without the use of an auxiliary instrument to break the joint.
The term "peelability" in this application refers to the sealed area of the adhesive where the adhesive is joined to a substrate. The adhesive peelability is measured by the degree of strength applied. Such hot melt adhesive peelability can be measured by ASTM testing method D-1876, (T-Peel test) for 180.degree. peelability and alternatively by ASTM testing method D-903 for 180.degree. peelability.
The invention relates to blends and a method for making a blend usable as a hot melt adhesive in a packaging application. This invention relates to laminates prepared with these hot melt adhesives and which are characterized by a nearly constant peel strength over an extended heat seal or application temperature. The blends, films and/or laminates of the present invention permit the manufacture of a more consistent finished product, having a seal of predictable and constant peel strength, in spite of inevitable variations in the heat seal temperatures or application temperature used in the production of such packages.
In the past, many varieties of thermoplastic materials have been employed in the manufacture of films capable of forming peelable seals. See, for example, U.S. Pat. No. 4,189,519, to American Can, which discloses a blend for producing a peelable heat seal comprising (1) about 50 to 90 percent by weight of a copolymer of about 80 to 96 percent by weight ethylene and about 4 to 20 percent by weight of an ethylenically unsaturated ester, and (2) about 10 to 50 percent by weight of a crystalline isotactic polybutylene. While capable of forming a peel seal, the film of the '519 patent comprises polybutylene as a minor component. The blend of '519 bonds to high density polyethylene (HDPE) without the use of adhesive, and will not bond to polypropylene without an adhesive.
U.S. Pat. No. 3,900,534 to Mobil Oil Corporation discloses thermoplastic shrink films with good heat seal characteristics and good optical properties, however, '534 does not address the need for a peel seal film.
U.S. Pat. No. 3,879,492 to UCB S.A. Belgium discloses blends consisting of polybutylene, styrene-butadiene copolymer, low density polyethylene, high density polyethylene, and polyisobutylene.
U.S. Pat. No. 4,539,263 to E.I. DuPont de Nemours & Co. discloses peel seals based on propylene/.alpha.-olefin copolymer. U.S. Pat. No. '263 does not appear to provide disclosure directed toward polybutylene polymers.
U.S. Pat. Nos. 4,665,130 and 4,666,778 disclose blends of polybutylene and EVA (or polyethylene) and polypropylene with the polypropylene being less than 15 weight percent in the blend. However, none of these references teaches the novel invention.
U.S. Pat. No. 3,573,240 describes hot melt adhesive compositions for hard cover book binding. At column 4, lines 1 through 7, '240 teaches that minor amounts i.e., up to about 5% by weight of alpha olefin comonomers such as ethylene and propylene may be present in the butene-1 polymerization system without any substantial loss of the desirable properties displayed by the resultant, essentially homopolymeric system. The '240 patent also recites, in column 2, lines 61 through 63, that the book binding hot melt adhesives display good heat stability and rapid setting speeds. The '240 patent does not disclose a peelable hot melt adhesive, even though up to about 5% by weight of ethylene may be added to a substantially butene-1 polymerization system. Thus, '240 is in opposite from the teachings of the present invention which describes the use of a blend of an ethylene homopolymer or copolymer with a modified butene-1 homopolymer or copolymer to form a peelable bond.
Other patents of interest include U.S. Pat. No. 4,198,327 to Matsumoto which describes a composition having improved adhesion that is made up of a Component A (which is 70 to 99 parts of a crystalline polyolefin which has been grafted with a monomer selected from the group consisting of unsaturated carboxylic acids and their anhydrides, esters, amide imides, and metal salts with the grafting monomer in an amount of 0.0001 to 3% by weight) and a second Component (B) which is 1 to 30 parts by weight of a hydrocarbon elastomer wherein the hydrocarbon elastomer is either ethylene/propylene rubber, ethylene-1-butene rubber, butyl rubber, butadiene rubber, sytrene/butadiene rubber, ethylene/butadiene rubber, or isoprene rubber.
Also of interest are two patents issued to Inoue, U.S. Pat. Nos. 4,619,972 and 4,440,911. U.S. Pat. No. '972 describes a modified polymer composition comprising elastomers blended with not more than 50 parts by weight of a crystalline polyolefin that is prepared by reaction of the polymer with an unsaturated carboxylic acid or its acid anhydride. The elastomers of this patent include ethylene/butene-1 copolymer, ethylene/propylene copolymer, a styrene/butadiene copolymer or mixtures thereof. The crystalline polyolefin of this invention includes homopolymers of olefins or copolymers of different kinds of olefins such as high density polyethylene, medium density polyethylene, low density polyethylene, crystalline ethylene/propylene copolymer, crystalline ethylene/butene-1 copolymer, polypropylene, crystalline propylene/ethylene copolymer, polybutylene-1, and poly(4-methyl-pentene-1). Mixtures of two or more of those polymers or copolymers can be used. U.S. Pat. No. '911 describes a fabricated composition that comprises linear low density copolymers of ethylene and an alpha-olefin where the alpha-olefin is selected from the group which includes butene-1, hexene-1, methylpentene-1, and octene-1 and comprises between 3 and 14 weight percent of the copolymer where the copolymer is reacted with 0.01 to 3 weight percent of an unsaturated carboxylic acid or a derivative thereof.
Additionally, U.S. Pat. No. 4,298,712 to Machonis describes adhesive composition which are related to the present invention. Additional references include U.S. Pat. Nos. 3,658,948 to McConnell; 3,868,433 to Bartz; and Japanese Patent No. 0179543. These are patents that describe inventions that fall within the general area of the present invention. Also, U.S. Pat. No. 3,896,067 to Kosaka, and U.S. Pat. No. 4,602,056 to Waniczek, as well as two Japanese Patents, Patent No. 2010107 to Mitsubishi and Patent No. 1181882 to Mitsui, are of interest. The Kosaka patent discloses a hot melt adhesive containing a maleic anhydride grafted EVA copolymer and a polyolefin. The Waniczek patent shows grafted EVA in hot melt adhesives. The Japanese references pertain to blends of modified butene polymers with ethylene polymers and are representative of the state of the art.
The present invention, directed to a hot melt adhesive made from specific modified ethylenic copolymer and specific butene-1 homopolymers with or without other components, appears to lie outside the scope of the known art.
Hot melt adhesives with long open time good bonding to cold metals are made of blends of a maleic anhydride modified copolymer of butene-1 and ethylene, an aliphatic, substantially non-polar resin, a antioxidizing agent and, optionally, microcrystalline wax, a block copolymer and atactic polypropylene are described in U.S. Pat. No. 4,554,304. This patent discloses a variety of modified butene-1 homopolymers and copolymers useful within the scope of the present invention, as well as methods for preparing these modified butene-1 polymers. Modified ethylene copolymer and homopolymer usable within the scope of the present invention are not taught in this disclosure.
The instant invention recognizes that conventional multilayer peel seal films or sheets are comprised of substrates and peelable sealings wherein the sealants are not hot melt type of adhesives. The present invention teaches a novel hot melt adhesive blend, a laminar structure and a method of making a packaging film system which is capable of peelability. The instant invention has, as an object, a peelable system which avoids delamination between layers when the sealed layers are pulled apart. In some embodiments, a tackifying resin is added to help bind together incompatible substrates.
It should be noted that the present invention has been developed to provide a blend of two non-compatible polymers to form a bond with a controllable cohesive strength.
Several terms are used throughout the specification. "Noncompatible polymers" are defined as polymers which, when blended or mixed, separate into two separate phases. "Controllable Cohesive Strength" refers to an adhesive wherein the cohesive strength can be varied or controlled by changing the ratio of noncompatible components.